The present invention relates to a method for forming a semiconductor device, and more particularly to a method for forming electrodes on mesa structures of a semiconductor substrate.
A typical prior art reference will be described with reference to FIGS. 1A to 1D. As illustrated in FIG. 1A, a photoresist film 1 is selectively formed on a silicon substrate 2 so that the photoresist film is used as a mask for etching a selective part of the silicon substrate 2. As a result, on the silicon substrate 2, there is formed a rectangular-shaped convex portion which serves as a mask alignment mark 3 in a later process.
With reference to FIG. 1B, the photoresist film 1 is removed from a surface of the silicon substrate 2. A photoresist film 4 is formed on the surface of the silicon substrate 2 by an alignment with reference to the mask alignment mark 3. The photoresist film 4 is used as a mask to etch the silicon substrate 2, thereby an isolation mesa structure 5 is selectively formed on the surface of the silicon substrate.
With reference to FIG. 1C, the photoresist film 4 is removed from the surface of the silicon substrate 2. After this, a photoresist film 6 is formed on the surface of the silicon substrate 2 by an alignment with reference to the mask alignment mark 3. The photoresist film has openings over the mesa structure 5 of the silicon substrate 2. The photoresist film 6 is used as a mask for an evaporation of an electrically conductive material 7. As a result, the conductive material 7 is deposited not only on the photoresist film 6 but also on exposed surfaces of the silicon substrate 2.
With reference to FIG. 1D, the photoresist film 6 covered by the conductive material 7 is removed from the surface of the silicon substrate 2. As a result, the conductive material remains only on the exposed silicon surface which had not been covered by the photoresist film 6. The remaining conductive material is subjected to a lift-off treatment to form electrodes 8 on the mesa structure 5 of the silicon substrate 2.
The conventional method for forming the electrodes on the mesa structure of the silicon substrate is, however, engaged with a serious problem regarding significant deterioration of the accuracy of mask alignment when the photoresist films 4 and 6 are provided on the surface of the silicon substrate 2. In fact, the photoresist films 4 and 6 tend to be slightly displaced from a predetermined exact position on the silicon substrate. Such displacements of the photoresist films 4 and 6 result in a displacement of the electrodes from predetermined positions on the mesa structure 10 of the silicon substrate 2 as illustrated in FIG. 2. Such a semiconductor device is, in fact, unacceptable.
Another conventional method will be described with reference to FIG. 3A to 3C. With reference to FIG. 3A, a photoresist film 1 is selectively formed on a GaAs substrate 2 so that the photoresist film 1 is used as a mask for etching a selective part of the GaAs substrate 2. The etching is carried out by use of a phosphor-containing etchant. As a result, on the GaAs substrate 2, there is formed a rectangular-shaped convex portion which serves as a mask alignment mark 3 in a later process.
With reference to FIG. 3B, the photoresist film 1 is removed from a surface of the GaAs substrate 2. A photoresist film 4 is formed on the surface of the GaAs substrate 2 by an alignment with reference to the mask alignment mark 3. The photoresist film 4 is used as a mask to etch the GaAs substrate 2 by use of a phosphor-containing etchant, thereby an isolation mesa structure 5 is selectively formed on the surface of the GaAs substrate 2.
With reference to FIG. 3C, the photoresist film 4 is removed from the surface of the GaAs substrate 2. Thereafter, a photoresist film 6 is formed on the surface of the GaAs substrate 2 by an alignment with reference to the mask alignment mark 3. The photoresist film 6 has an opening over the mesa structure 5 of the GaAs substrate 2. The photoresist film 6 is used as a mask for an evaporation of an electrically conductive material 7. As a result, the conductive material 7 is deposited not only on the photoresist film 6 but also on exposed surfaces of the GaAs substrate 2.
With reference to FIG. 3D, the photoresist film 6 covered by the conductive material 7 is removed from the surface of the GaAs substrate 2, As a result, the conductive material 7 remains only on the exposed GaAs surface which had not been covered by the photoresist film 6. The remaining conductive material is subjected to a lift-off treatment to form electrodes 8 on the mesa structure 5 of the silicon substrate 2.
The conventional method for forming the electrodes on the mesa structure of the silicon substrate is also engaged with the same problem of significant deterioration of the accuracy of mask alignment when the photoresist films 4 and 6 are provided on the surface of the silicon substrate 2. In fact, the photoresist films 4 and 6 tend to be slightly displaced from a predetermined exact position on the silicon substrate. Such displacements of the photoresist films 4 and 6 result in a displacement of the electrode 9 from predetermined positions on the mesa structure 5 of the silicon substrate 5 as illustrated in FIG. 4. Such a semiconductor device is, in fact, unacceptable.
Under the above circumstances, it has been necessary to develop a quite novel method for forming electrodes on a mesa structure of a semiconductor substrate.